The present invention relates to a multi-beam laser light source used in either a multi-beam type laser beam printer, or a multi-beam type optical disk.
The present invention relates to a multi-beam semiconductor laser array employed as a light source in multi-beam type laser beam printers and optical disk apparatuses.
Conventionally, it is known in the field to use a multi-beam laser light source to simultaneously scan plural lines to equivalently increase the scanning speed. In another conventional technique, the semiconductor laser array is used as a multi-beam laser light source and this semiconductor laser array is combined with interlaced scanning in order to reduce the pitch of the scanning lines. This technique has been reported in, for instance, K. Minoura, M. Suzuki, and S. Miyazawa, SPIE, Vol. 1079, page 462 in 1989, and Japanese patent publication No. 1-45065 in 1989. The Inventors or Applicants have reported that, in a multi-beam laser printer employing a semiconductor laser array as a light source, there is an inverse proportional relationship between the array interval "x" of the semiconductor laser array and the divergence angle .theta. of the laser beam projected from the respective semiconductor laser elements along the array arranging direction (see Extended Abstracts (The 52nd Autumn Meeting, 1991); The Japan Society Applied Physics by Ota, Ito, and Tatsuoka, 11p-ZM-19, and Japanese Patent application No. 3-0158608 filed by the Applicants).
Concretely speaking, the following relationship can be satisfied between the array interval "x" of the semiconductor laser array and the divergence angle .theta. along the array arranging direction: EQU x=2A.lambda.i/{k.pi.sin (.theta./2)} (1)
where "A" denotes a spot divergence (apodization) coefficient determined by an aperture of a major image forming optical system, ".lambda." denotes a wavelength of the laser beam, "i" denotes an interlaced period (scanning order), and "k" denotes a spot diameter correction coefficient. The typical values of the above-described parameters are, for instance, A=1.98, .lambda.=0.78 micrometers, i=1 (corresponding to such a non-interlaced scanning operation), and k=1.5.
As apparent from the foregoing description, if the divergence angle ".theta." of the laser beam is small, then the array interval "x" of the semiconductor laser array may be made wide.
Generally speaking, the divergence angle of the laser beam of the semiconductor laser is equal to approximately 8.degree. to 15.degree. (defined by 1/e.sup.2 of center intensity) along the parallel direction with respect to the junction plane, and it is rather difficult to greatly change this divergence angle. Assuming now that the divergence angle ".theta." of the laser beam along the array arranging direction is 12.degree.. When these typical values of the respective parameters are substituted into the equation (1), the array interval "x" is calculated as 6 micrometers. An array interval of a semiconductor laser array may be manufactured on the order of 10 micrometers in accordance with the current manufacturing level. However, since array intervals narrower than approximately 10 micrometers hardly can be manufactured, interlaced scanning techniques are necessarily required.
On the other hand, such a method has been filed by the Applicant in Japanese patent application No. 3-227532 in 1991 in which the micro lens is combined with the semiconductor laser to reduce the virtual divergence angle of the laser beam, and therefore the intervals of the imaging spots on the image forming plane (photoreceptor surface plane) can be effectively and closely spaced with each other even when the semiconductor laser array having a wide array intervals is employed.
As represented in FIG. 7, in this conventional structure a micro lens array 2 is provided, corresponding to the respective semiconductor laser elements 1a and 1b of the semiconductor laser array 1. The laser beams emitted from the semiconductor laser elements 1a and 1b have passed through the respective micro lenses of the micro lens array 2, and further have passed through the objective lens 25, and thereafter are focused onto the focal plane (photoreceptor surface plane) P3.
In the optical system shown in FIG. 7, the positions of the projection points of the laser beams are moved to the second plane P2 located behind the first plane P1 where the position of the light source is actually located, as viewed from the objective lens 25, and also the divergence angle .theta..sub.1 of the laser beams along the array arranging direction is reduced to .theta..sub.2.
It should be noted in FIG. 7 that "a" indicates a distance between the major plane of the micro lens array 2 at an image side and the first plane P1; "b" indicates a distance between the major plane of the micro lens array 2 at the image side and the second plane P2; "r.sub.1 " indicates an array interval of the semiconductor laser array 1; "r .sub.2 " indicates an array interval of the micro lens array 2; "f.sub.1 " indicates a distance between the second plane P2 and the objective lens 25; and "f.sub.2 " indicates a distance between the objective lens 25 and the focal plane (photoreceptor surface plane) P3.
On the other hand, a conventional method has been proposed in which a plurality of toroidal lenses are provided which are used in the Pyramidal Error Correction Optics of the laser beam scanning apparatus with using the polygon, so that the imaging spots on the focal point plane (sensitive body plane) are closely spaced (see Japanese Laid-open Patent Application No. 59-15219, laid open in 1984). This is schematically illustrated in FIG. 8, which shows that by expanding the light path, the laser beams projected from the respective semiconductor laser elements of the semiconductor laser array 1 are converted into parallel beam light by the collimator lens 21, the optical axes of the respective laser beams are directed toward the inside by the cylindrical lens 22, and the respective laser beams are reflected at the mirror surface of the polygon scanner 23. Furthermore, the reflected laser beams pass through the toroidal lenses 24a to 24c provided in relation to those laser beams, and then are focused onto the focal point plane (photoreceptor surface plane) P3 by the objective (f-.theta.) lens 25.
In the prior art shown in FIG. 7, the divergence angle .theta. of the laser beams is reduced, whereas in the prior art shown in FIG. 8, the optical paths of the laser beams are varied every laser beam by providing a plurality of toroidal lenses 24a to 24c between the polygon scanner 23 and the objective (f-.theta.) lens 25.
However, the above-described conventional methods have the following problems.
With respect to the interlaced scanning operation, since the respective laser beams must be independently modulated by the image (picture) signals having predetermined time relationships, complex controls are required for the image signals. Since the intervals of the adjoining imaging spots on the image forming plane (photoreceptor surface plane) are wide when this interlaced scanning operation is carried out, high mechanical precision is required for the scanning optical system (see the above-described Japanese patent application No. 3-158608), and the scanning lines are easily distorted in accordance with the f-.theta. correction (see the above-explained publication of SPIE Vol. 1079, page 462, by K. Minoura, M. Suzuki, and S. Miyazawa in 1989).
Also, in one conventional method for combining the micro lens array and the semiconductor laser as shown in FIG. 7, a change is required in designing the major image forming optical system in accordance with the reduction of the divergence angle of the laser beam, as compared with that used in the single laser beam printer. Accordingly, there is a problem that, for instance, the beam expander must be additionally required in the major image forming optical system (see the above-explained Japanese patent application No. 3-227532).
Further, in the other conventional method for providing a plurality of toroidal lenses to change the optical paths of the laser beams, the required plurality of toroidal lenses having complex structures causes high cost and, moreover, the alignment adjustments must be performed for each of these toroidal lenses.
Further, conventionally, multi-beam type laser beam printers and optical disk apparatuses have been known in the art. For instance, one multi-beam type laser beam printer is described in "A study or laser scanning systems using a monolithic arranged laser diode, written by K. Minoura, M. Suzuki and S. Miyazawa, Proceedings SPIE, Vol. 1079, pages 462-474 in 1989. Also, one multi-beam type optical disk apparatus is disclosed in "Highspeed Magneto-Optical Disk Drive With Employment Of 4-Beam Optical Head," written by Nakagome, Kogaku Vol. 20, No. 11, pages 741 to 742 in 1991.
Very recently, such a multi-beam semiconductor array laser wherein laser elements are positioned adjacent to each other in an interval of 10 micrometers has been developed as the light source for these multi-beam type laser beam printer and optical disk apparatus (see Japanese Laid-open Patent Application No. 2-39583 opened in 1991, and "Properties of closely spaced independently addressable lasers fabricated by impurity-induced disordering" by R. L. Thornton et al., Applied Physics Letter 56 (17), pages 1623-1625 in 1990).
Another method for essentially closely spacing spot intervals on a focused plane by combining the multi-beam semiconductor laser array with the micro lens array with employment of such a multi-beam semiconductor array having wide array intervals, has been described in Japanese Laid-open Patent Application No. 3-227532, laid open in 1991 and filed by the Applicant.
In FIG. 15, there is shown the first method described in the above-mentioned Japanese Laid-open Patent Application No. 3-227532. Laser beam light projected from a plurality of laser beam optical sources LS at an divergence angle .theta..sub.1 (defined by 1/e.sup.2 of center intensity), is processed by lenses L.sub.2 provided dependent to the respective laser beam optical sources LS such that the divergence angle .theta..sub.1 is reduced to another divergence angle .theta..sub.2 (defined by 1/e.sup.2 of center intensity). As a result, the positions of the laser beam light sources LS are equivalently present at the virtual optical source plane P1 of FIG. 15, as optically viewed from the imaging lens L1. There is a conjugate relationship between the virtual optical source plane P1 and the image forming plane P2 by way of the imaging lens L1. It is assumed that a diameter of an imaging spot of the laser beam light on the image forming plane P2 is "d2" when lateral magnification .beta. is equal to f.sub.2 /f.sub.1. It should be noted that symbol f.sub.1 denotes a distance between the light source plane P1 and the imaging lens L1, and symbol f.sub.2 indicates a distance between the imaging lens L1 and the image forming plane P2. Also, it is assumed that another diameter of an imaging spot of the laser beam light on the image forming plane P2 is d1 when the image is formed at the lateral magnification .beta.=f.sub.2 /f.sub.1 by employing the image forming lens L1, but no image forming lens L2. Comparing the above-explained two optical systems with each other, one diameter d2 of the imaging spot is larger than the other diameter d1 of the imaging spot, though the intervals "r" of the imaging spots are equal to each other.
In accordance with the method as shown in FIG. 15, the divergence angle .theta..sub.1 of the laser beams is reduced to .theta..sub.2 by the projection points of the laser beams (precisely speaking, positions between beam waists) are positioned inside the focal distance f.sub.1 of the imaging lens L1, and the virtual projection points of the laser beams, namely the virtual optical source plane P1 are moved in the left direction, as viewed in FIG.15.
FIG. 16 represents the second method described in the specification of the above-mentioned Japanese Laid-open Patent Application No. 3-227532. The laser beam light projected from the arrayed semiconductor laser element 1 is converted into geometrically and optically parallel light by way of the arrayed micro lens 6. However, the converted parallel light wave-optically has a certain divergence angle .theta..sub.2. This divergence angle .theta..sub.2 is smaller than the divergence angle .theta..sub.2 of the laser beam light projected from the semiconductor laser element 1. In this case, the optical system is designed in such a manner that the virtual projection points of the laser beams are present at the major plane of the micro lens 6.
As described in the above-explained specification, the spot diameter is increased, while the spot intervals on the image forming plane remain identical in accordance with the decrease in the divergence angle of the laser beam light. As a consequence, if the divergence angle of the laser beam light is made small, even when the array intervals are wide, the spot intervals on the image forming plane can be substantially closely positioned with each other by adjusting the magnification of the optical system.
However, the above-described conventional methods, as shown in FIGS. 15 and 16, have problems because since the multi-beam semiconductor laser array and the micro lens array are provided as an independent optical component, optically difficult alignment adjustments are required for these optical components.